The origin of worm-like bodies and their relationship with Birbeck granules is poorly understood. To clarify the origin of worm-like bodies and their relationship with Birbeck granules. Over 800 electron micrographs of histiocytic disorders and several appendage tumours were reviewed in order to check for worm-like bodies and Birbeck granules. Worm-like bodies were most often encountered in mildly- to moderately-proliferative histiocytic tumours. Birbeck granules were observed in more malignant conditions. Narrow endoplasmic reticulum (Nrer), which resembles worm-like bodies, was abundant in worm-like body-rich cells and coexisted with Birbeck rods. Nrer is thought to be one of the candidates that gives rise to worm-like bodies. An ultrastructural similarity exists between worm-like bodies (and octopus body formation) and the so-called "lupus erythematosus virus". The presence of Nrer is often concomitant with other organelle markers, and could be a candidate for the origin of worm-like bodies.